The invention is in the field of optics, manufacturing methods and processes for producing optical devices and optical devices so produced and being useful for performing operations on optical signals, including but not being limited to filtering, diffracting, discriminating and sensing.
Optical Bragg grating devices are used for performing many operations on optical signals, such as filtering, light diffraction, and sensing. Optical waveguide grating, in particular, do theses function while guiding and confining the light in the waveguide medium as well. A waveguide grating is normally formed on a waveguide in which at least one of its parameters is changed almost periodically along the length of the waveguide. The most commonly perturbed physical parameter in waveguide grating structures is the refractive index. The waveguide structure with periodically perturbed refractive index can be used as an optical filter in which an optical signal is reflected back by the grating structure at the Bragg wavelength defined by:
xcexB=2neffxcex9
where xcexB is the Bragg resonance wavelength and neff is the average effective index of the waveguide, and xcex9 is the longitudinal period of refractive index change along the waveguide. A variety of optical wavelength band reflection/rejection or transmission filters can be designed consequently to perform the desired functions. The optical filter can be designed to have very narrow, that is less than 0.1 nm line width, or to have relatively wide band filters with desired transmission reflection wavelength characteristics in the order of few tens of nm line-width. For instance they can be used for separating one particular band of the optical signal in wavelength division multiplexing (WDM) optical transmission system or as dispersion compensators in long haul transmission systems.
An efficient and popular method of imprinting gratings on waveguides is to use photosensitive waveguides whose refractive index can be changed by exposure to radiation of a particular nature, for example ultraviolet (UV) electromagnetic radiation. Usually a grating is imprinted by exposing the waveguide under an interferometric pattern of ultraviolet sources using holographic or phase mask methods. Imprinting grating by holographic method is described in an article entitled, xe2x80x9cFormation of Bragg Gratings In Optical Fiber By Transverse Holographic Methodxe2x80x9d, by G. Melts et al., published in 1989 in Optics letter Vol. 14, No. 15, at pages 823-825. In the holographic, or interferometric method, waveguide grating is formed by exposing the piece of fiber to an interfering pattern of two ultraviolet beams of light to produce a standing wave to which the waveguide is exposed. The refractive index of the waveguide is locally and periodically changed in the exposed area. This grating fabrication approach requires a laser with high spatial and temporal coherence, and is highly sensitive to alignment and vibration during production. These requirements are more strict in the case of a chirped grating in which the period of grating pitches must be changed along the waveguide. Imprinting grating using a phase mask method is described, for example, in an article entitled, xe2x80x9cBragg Gratings Fabricated In Monomode Photosensitive Optical Fiber By UV Exposure Through A Phase Maskxe2x80x9d by K. O. Hill et al. published in 1993 in Applied Physics Letters, Volume 62, No. 10, pages 1035-1037. The method is also published in the U.S. Pat. No. 5,367,588, issued to K. Hill et al on Nov. 22, 1994 and entitled, xe2x80x9cMethod of Fabricating Bragg Grating Using A Silica Phase Grating Mask and Mask Used By Samexe2x80x9d. In this approach, a phase mask splits the beam into several diffractive orders that interfere to create the required pattern. The phase mask method is less sensitive to spatial coherence and alignment. It can also be used to produce chirped gratings. However it still needs proper optical alignment, careful control of the space between the phase mask and the waveguide, with a precise control of waveguide motion under the phase mask at the same time. In the U.S. Pat. No. 5,837,169, xe2x80x9cCreation Of Bragg Reflective Gratings In Waveguides,xe2x80x9d by H. N. Rourke, Issued No. 1998, there is disclosed a method of writing long fiber grating at several stages using a number of phase masks that have an alignment part which is a replicate of the portion of the writing part of the adjacent mask. Careful motion adjustment must be made to align the consequent masks and keep the writing conditions the same for each stage of writing gratings. A number of research papers and patent disclosures, some of which are listed herein, propose new optical devices using Bragg grating or disclose improved methods of imprinting Bragg grating based on the two above mentioned methods. Nevertheless, grating fabrication method using these approaches are still time consuming and unpredictable due to the required mechanical motion accuracy and stability. This results in low yield in fabrication and therefore a high manufacturing cost. Therefore there is a need for alternative methods of manufacturing Bragg grating devices on waveguides that is suitable for volume manufacturing.
In accordance with an aspect of the invention, a method for manufacturing an optical circuit includes provision of an optical waveguide being carried by a surface of a substrate, the optical waveguide having a light transmission property which is alterable in response to radiation of a predetermined nature. A first mask is applied to the surface of the substrate, the first mask being non-transparent to the radiation and includes registration means and a radiation transparent aperture defined therein for permitting a predetermined area of the optical waveguide to be exposed to the radiation. A second mask is affixed over the first mask in alignment with the registration means. The second mask includes a plurality of ports defined therein, each of the ports overlapping the aperture and being transparent to the radiation. A predetermined amount of the radiation is directed at the second mask, whereby areas of the optical waveguide are exposed via said ports and the aperture such that a light transmission property in each of said areas is altered to effect manufacture of the optical circuit.
Advantageously the second mask is removed from the optical circuit and is useful in the fabrication of similar optical circuits.
In accordance with an another aspect of the invention, a process for effecting post manufacture adjustment of an optical circuit having been formed in an optical waveguide having a light transmission property being alterable by exposure to radiation of a predetermined nature, includes the steps of: a) determining an operating characteristic of the optical circuit by transmitting light energy in a spectrum in which the optical circuit is intended to be operable via said optical circuit and receiving and detecting any of said light energy having traversed the optical circuit; b) determining if the operating characteristic is adjustable toward an operating parameter standard by adjusting the light transmission property of at least one predefined portion of the optical circuit, and if YES; c) selectively exposing the at least one predefined portion of the optical circuit to a beam of radiation while continuing to perform step a) and if the determination in step b) becomes NO, stopping the exposure in step c).
An apparatus in accordance with an another aspect of the invention, provides for post manufacture processing of an optical circuit including an optical waveguide having a light transmission property which has been altered by exposure to radiation. The apparatus includes a table for mounting the optical circuit and a beam source for providing a beam of radiation of a predetermined nature for impinging upon an optical circuit mounted on the table. A source of light energy in a spectrum in which the optical circuit is intended to be operable is connectable for transmitting light to the optical circuit when it is mounted on the table. A detector is connectable to receive light energy from the optical circuit when mounted on the table. The detector generates indications representative of received light energy. A controller is dependent upon the indications of received light energy from the detector and a data base peculiar to a particular design of an optical circuit, for selection at least one portion of an optical circuit mounted on the table and directing the beam of electromagnetic radiation from the beam source means upon a selected portion of the mounted optical circuit.
In accordance with yet another aspect of the invention an optical waveguide assembly comprises a circuit substrate having a substantially planar surface and an optical waveguide being imbedded in the substantially planar surface of the circuit substrate. The optical waveguide has a light transmission property being alterable in response to radiation of a predetermined nature. A mask overlies the substantially planar surface of the substrate and the optical waveguide. The mask includes an aperture therein for defining an area of the waveguide which is accessible for exposure to said electromagnetic radiation. The mask also includes a registration means to provide for alignment of the substrate with processing apparatus useful in the manufacture of an optical circuit.
In one example the optical waveguide assembly includes an optical circuit being contained along and within a portion of the optical waveguide in a substrate overlaid with a first mask and having been manufactured by exposure of areas of the optical waveguide, via a grating mask registered with the first mask, to electromagnetic radiation of an incoherent nature within the ultraviolet spectrum. After manufacture the grating mask is removed and an aperture in the first mask defines an area of said portion of the waveguide which is available for post manufacture modification by controlled exposure to said electromagnetic radiation.